attempt to concatenate nill and string help!

ok so i have this program im working on (data over Redstone) and i keep (randomly) getting this bug
please help, i cant figure out what is going on! to get the error you need to connect to computers with Redstone around the back and run the client on one and the pulse on the other and wait (sorry i dont know
how else to trigger the error)

(pastebin)
Client: YUX7GTf5
Pulser: https://pastebin.com/iez6Qdzm

help is very appreciated, this project isn't finished but i need to fix this bug before i can continue

Your error should contain a line number to tell you where the problem is.

From looking at the code my guess is line 34. Did you know Lua is 1-indexed and 0-indexed? letters[1] == "a", letters[2] == "b" etc and letters[0]== nil because it has not been explicitly defined.